The present invention concerns a method of producing and treating a sheet suited to be used as a component or as a part of a component in a fuel assembly for a nuclear light water reactor, which method comprises the following steps:    a) producing a sheet of a Zr-based alloy by forging, hot rolling and cold rolling in a suitable number of steps, wherein said alloy contains at least 96 weight percent Zr and is of such a kind that the sheet is suitable to be used for said component,    b) carrying out an α+β quenching or a β quenching of the sheet when the sheet has been produced to a thickness which is equal to the final thickness, or at least almost equal to the final thickness, of the finished sheet,    c) heat treatment of the sheet in the α-phase temperature range of said alloy,wherein step c) is carried out after steps a) and b) have been carried out.
The invention also concerns a use and devices which will be described below.
The above described method may for example be used for producing channel boxes for fuel assemblies for a boiling water reactor (BWR). Such a method is for example known through WO-A1-97/40659.
Below first an example of a known fuel assembly for a BWR will be described with reference to FIGS. 1-3.
FIG. 1 thus schematically shows a fuel assembly for a BWR. The fuel assembly comprises a channel box 2 (which here is only shown to the right in the figure). Inside the channel box 2 a number of fuel rods 3 are arranged. The fuel rods 3 extend from a top plate 5 to a bottom plate 6. The fuel rods 3 consist of cladding tubes which contain pellets with nuclear fuel material. In the figure a number of pellets 4 are symbolically shown. At the top, the fuel rods 3 are provided with end plugs 8. The fuel rods abut against the lower side of the top plate 5 with the help of coiled springs 9. A plurality of spacers 7 are arranged for holding the fuel rods 3 at a distance from each other. The fuel assembly is long and thus has a longitudinal direction which is here indicated with a central axis 10. The fuel assembly may often comprise a water channel which usually extends over substantially the whole length of the fuel assembly and which enables a flow of non-boiling water up through the fuel assembly.
FIG. 2 shows schematically a cross-section of a fuel assembly for a BWR. This cross-section shows that the fuel assembly comprises a central water channel 12 with a square cross-section and four smaller water channels 14.
FIG. 3 shows schematically a cross-section of another construction of a fuel assembly for a BWR. This cross-section shows that the fuel assembly in this case only comprises one water channel 16 which has a square cross-section.
Both the above mentioned channel box 2 and the water channels 12, 14 and 16 are often produced of sheet materials which are formed and welded together in a suitable manner such as is well known to a person with knowledge within the field. Concerning the channel box 2, this can be produced by producing two sheets. Each sheet is bent such that a U-shaped profile is achieved. These U-shaped profiles can then be welded together such that a channel box 2 with a square cross-section is obtained. FIG. 4 indicates schematically a cross-section of two such U-shaped profiles before they have been joined and welded together. FIG. 5 shows the channel box 2 when the two U-shaped profiles have been welded together. The weld seams are here indicated by 18.
Channel boxes and water channels for fuel assemblies are usually produced in different Zr-based alloys which are well known to a person with knowledge within the field. For example, the well-known alloys Zircaloy-2 and Zircaloy-4 may be used.
In the very particular environment that a nuclear reactor constitutes, the components that form part thereof have to meet many requirements. A very large number of suggestions for the selection of material and for methods of production of components for fuel assemblies for nuclear reactors have therefore been produced. Even small modifications in the composition of alloys or in parameters of production can have a large importance for the properties of the components.
The above mentioned WO-A1-97/40659 describes a method of producing sheet material of a Zr-based alloy for producing channel boxes for fuel assemblies for a BWR. According to the described method, a sheet of the Zr-based alloy is produced by forging, hot rolling and cold rolling in a number of steps. Between the rolling steps a heat treatment may be carried out. When the sheet has been produced to the final or almost the final dimension it goes through a β quenching. Through the β quenching, the properties of the sheet are improved. Among other things, the corrosion properties are thereby improved. Furthermore, through the β quenching a more randomised texture of the crystal grains are achieved, which works against the tendency of the sheet to be deformed in particular selected directions. With a randomised texture is meant that the crystal grains are directed randomly in different directions. With a non-randomised texture is thus meant that the crystal grains tend to be directed in one or some particular directions to a larger extent. A sheet with a non-randomised texture therefore tends to be deformed in particular selected directions.
In this context it can be noted that the used Zr-alloys exist in an α-phase at lower temperatures (for example at room temperature). In the α-phase, the crystal structure of the material is hcp. At higher temperatures (for example for Zircaloy-4 above about 980° C.) the alloy exists in β-phase. In this phase, the crystal structure is bcc. At a temperature which for example for Zircaloy-4 is between 810° C. and 980° C. the alloy exists in a mixture of α-phase and β-phase, a so-called α+β-phase.
According to the above mentioned WO-A1-97/40659, the sheet may go through a heat treatment in the α-phase temperature range after said β quenching. Thereby the corrosion properties of the sheet are further improved.